What goes into a baseball slash line?
I’m trying to slowly ease my way into understanding non-counting baseball statistics. Many of them are pretty straightforward: on-base percentage for example, is intuitive, and some of the more advanced stats like OPS+ or WAR are also easily understandable when you read a description of how they’re put together. The problem, though, is that I don’t really get how to calculate many of the rudimentary or advanced stats, and even though I’ve read how to calculate slugging percentage a couple of times, I forget how it’s done!
So I thought it would be useful to teach myself, by pretending to explain to an imaginary reader, how various stats are computed and to start with the most ubiquitous shorthand for position-player talent — the slash line.
The slash line, or triple slash line, consists of three statistics. The first is batting average (AVG), the second is on-base percentage (OBP), and the third is slugging percentage (SLG). The three-letter shorthands are how these three stats are referred to when included in the formulas for more advanced stats, so it’s useful to know how they’re habitually shortened.
Batting average is a measure of hits per at bat. Pretty simple! But what’s an at-bat (AB)? It’s not the same thing as a plate appearance (PA) — because that’s just any time a player gets up to the plate. In baseball terms, the batter gets charged with a PA anytime they make an out or get on base (either through hitting, walking, reaching on interference or getting hit), but isn’t charged with one if, while they’re in the batter’s box, a runner on base get’s caught stealing or otherwise thrown out, or if the winning and walk-off run comes in from third base because of a wild pitch, passed ball, or balk. They might also not be charged with a PA if they were replaced mid at-bat with less than two strikes by a pinch hitter. An AB, then, is PA minus a couple things — walks, getting beaned, sacrifice hitting (bunting or hitting a fly ball), or reaching base because of defensive interference.
So batting average is number of hits/number of at bats. If I get 1 hit in 2 at bats, my batting average is .5. Baseball uses three digits after the decimal point (so my average is expressed as .500) to make it easier, I presume, to make fine-grained distinctions between players. It’s easier to see who’s better between a guy who hits .250 and .299 if you can see to the tenths and hundredths places.
Next is on-base percentage. This includes in its numerator hits, hit-by-pitches, and walks. It’s not calculated as a percentage of at bats, though, but rather as a percentage of at bats + hit by pitches + walks +sacrifice flies. It doesn’t include defensive indifference or interference, but generally all other ways of getting on base are here in the denominator. On base percentage measures how well you get on base in any possible skill-based way (defensive indifference and interference having less to do with a batter’s skill than with the lack thereof of the defence, or with the wackiness taking place in the game).
The last stat in the slash line is slugging. This is a deceptively simple stat, calculated as the number of bases/AB. But what does the number of bases mean? That’s actually not too complicated — it’s simply 1 base for a single, 2 bases for a double, 3 for a triple, and 4 for a home run. So this stat partly measures your ability to get multiple bases, or your ability to “slug”. This is the only one of the three slash lines that can end up being more than .999, and that’s because the numerator can be larger than the denominator — e.g. if you had a game where you got two homers in 4 ABs, you would have 8/4 or 2.000 as a slugging percentage.
So that’s how each of these things is calculated. ABG is your ability to get hits, irrespective of what kind of hits those are. OBP is your ability to get on base — by hitting, walking, or getting hit — and SLG is your ability to get quality hits, where a larger number represents more total bases per at bat.
But what the hell do these actually mean? What’s a good slash line? Does the slash line tell us everything we need to know to judge a hitter? I’ll get to it! Tomorrow.
